CONCRETE 131 



construction work made with it resembled a rock quarried 

 at Portland. It differs from the other kinds in that the 

 two materials necessary for cement making are obtained in 

 different places, carefully selected, ground, and mixed, then 

 burned and reground. As a result of this special selection 

 and mixing, Portland cement is best for general purposes, 

 as its action and strength are more uniform than those of 

 any of the other varieties. It has about three times the 

 tensile strength of ordinary natural cement. 



Manufacture and handling of cement. A cement-manu- 

 facturing plant is a large institution, requiring an expensive 

 investment in machinery for handling the raw material, 

 crushing, grinding, conveying, and large rotary burning 

 kilns. Provision is made for carefully sampling the materials 

 and for mixing them at the proper time and in the proper 

 proportions. The product is manufactured in all plants in 

 conformity to standard specifications. 



Cement was formerly packed for shipment in barrels, and 

 the term barrel became so common that it is still considered 

 the standard unit for measuring, although at the present 

 time practically all cement is packed and shipped in sacks. 

 One sack of cement weighs 94 pounds net. Four sacks of 

 Portland cement or three sacks of natural cement make a 

 barrel. For practical purposes, one sack of Portland cement 

 may be considered as 1 cubic foot. 



Cement in small quantities, if purchased from reliable 

 dealers, is satisfactory as a rule. If cement is stored, it 

 should be placed where it cannot get wet from above, and 

 particular attention should be given to placing planks or 

 other dry material under the sacks. Wet cement immedi- 

 ately begins to harden and will never set properly even 

 though it has been repulverized. 



AGGREGATE 



Aggregate. Aggregate is mineral material, such as sand, 

 gravel, crushed rock, and rock screenings, and consists of both 



